From my experience of 7 nieces and nephews, a walker is not essential for babies development. Something to push along would be great, but only once he's actually walking. I think what you guys are doing now is just great!
Jumperoo! DS loved his. Also a push-along walker or a ride-on. DS was happy to have his own cupboard in the kitchen, and bang on the pans with wooden spoons. Also, she could get some musical books, the ones where you push some buttons on the side and they make specific noises or play some melodies.
Having musical toys will help the child develop quicker. It has been proven that music stimulates the brain activity and increases learning, not to mention creativity. It is recommended to introduce musical toys to your child as early as possible. Having light up musical mobiles in the crib from the time of birth has been shown to lower stress and increase brain function. I bought my child toy wind instruments and a piano toy and every day I listen to music with him. I always introduce new toys into his play time and I never allow him to become bored or under stimulated.
DS is now 18 mOnths and didn't really "get" most toys until he was nearly 1. After the Jumperoo wasting space & looking ugly & DS wasn't really all that interested anyway I set a rule everything had to be foldable/putawayable each night.
This might not play well to your MIL's desire to spend money, but he has got lots of joy from Treasure Baskets. Lovely non-messy sensory play and easy to fill with a rummage through kitchen drawers, recycling bin & Wilkos/the Range for cheapy wrapping paper and the like.